As a result of the increasing popularity of the microwave oven, various food products have been introduced that are prepared and packaged in a fashion susceptible to so-called one-step cooking. The consumer need only place the food product in a microwave oven for a prescribed period of operating time, after which the package can be opened and the cooked or heated food eaten. The preparation of many of these food products requires the generation of auxiliary heat, i.e., heat other than that heat induced in the food product itself by operation of the microwave energy. For example, such food products include those that are prepared by browning or crisping the exterior surface. Such food products typically require the use of a susceptor when prepared in a microwave oven in order to generate the heat necessary to produce browning or crisping. A susceptor is a device that when placed in a microwave field absorbs microwave energy and converts it into heat. Additionally, the preparation of other food products, such as popcorn, require the use of a susceptor in order to bring the oil or grease ingredient to a desired temperature. To these ends, several devices have been previously proposed for the generation of such auxiliary heat.
U.S. Pat. No. 4,190,757-- Turpin, et al., discloses a microwave heating package that includes a susceptor for generating heat necessary to prepare food products such as pizza, french fries, hash brown potatoes, onion rings, etc. The susceptor is described as a composite or laminate consisting of an upper structural support member and a heating layer. The heating layer is said to include any suitable microwave absorptive lossy substance that will reach a temperature above 212.degree. C. when exposed to microwave energy. The heating layer is described as a relatively thin layer of paint having a binder that bonds or cements absorbent particles together. Although shown as a continuous layer, it is stated that the heating layer may not be continuous, i.e., it can be provided in two or more strips or bands or may include holes or openings. Among the absorbent particles suggested for use in the heating layer are carbon and graphite.
U.S. Pat. No. 4,230,924-- Brastad, et al., discloses material for packaging food to achieve microwave browning, which material includes a flexible dielectric substrate, in the form of a sheet of plastic, having a metallic coating. The metallic coating is preferably aluminum that has been vacuum evaporated onto one side of the substrate. The metallic coating is said to be subdivided into a number of metallic islands or pads with non-metallic gaps or strips therebetween. The metallic pads are said to generate heat in either of two modes of operation, namely, by creating a flow of current through the substrate, thus generating heat in the spaces between the islands or by permitting a portion of the microwave energy to pass through the islands and converting the remaining energy into heat. In the former mode of operation, the substrate is said to be paperboard. In the latter mode of operation the overall resistance of each island is said to be in the range from one ohm per square (1.OMEGA./sq.) to three hundred ohms per square (300.OMEGA./sq.) per island.
U.S. Pat. No. 4,434,197-- Petriello, et al., discloses a cooking liner laminate. The outer layers of this laminate are preferably polytetrafluoroethylene and are substantially transparent to microwave energy. The inner layers are also formed from polytetrafluoroethylene. However, semi-conductive or energy absorbing material such as colloidal graphite is said to be disbursed within the layer. Similarly, U.S. Pat. No. 4,518,651-- Wolfe, discloses a flexible composite material for use in microwave cooking packages. The composite material is said to include a porous dielectric substrate, substantially transparent to microwave radiation, and an electrically conductive coating on one surface of the substrate. The coating is said to include electrically conductive particles in a thermoplastic dielectric matrix. The electrically conductive particles, which can include carbon black and graphite, are first disbursed in the thermoplastic matrix. The thermoplastic matrix is thereafter applied to the porous substrate, for example, by means of a coating knife, and the resulting composite is subjected to preselected pressures and temperatures. Somewhat similar is the susceptor disclosed in European patent application No. 242,952-- Hua-Feng Huang, published Oct. 28, 1987, which is said to include a dielectric substrate, for example, polyethylene terephthalate, coated with a mixture of electrically conductive metal or metal alloy in flake form disbursed in a thermoplastic dielectric matrix. It is indicated that graphite in flake form can be utilized as the electrically conductive material.
U.S. Pat. No. 4,640,838-- Isakson, et al., discloses a means for automatically venting a vapor-type package, that means is said to include a susceptor having non-metallic microwave-absorbing particles in a non-metallic binder. The particles are said to include graphite and carbon black. In operation, the susceptor generates heat sufficient to "melt" an opening in the vapor-tight package.
U.S. Pat. No. 4,751,358-- Durand, discloses an enamel which is said to incorporate electrically conductive elements in the form of a metal powder. It is suggested to apply the enamel to a microwave receptacle by hot silkscreen printing or by the decalcomania process.
Each of the above described susceptors or inductive heating elements are relatively costly to produce and, in certain situations, require elaborate manufacturing processes. Consequently, a need still exists for a microwave susceptor that not only generates the desired heat, but which is also easy and economical to manufacture.